Designers of transmission systems for use in motor vehicles are required to implement a transmission gear selector lever having a minimum of four positions, or ranges, in a sequence of Park, Reverse, Neutral, and Forward Drive, as specified in the Federal Motor Vehicle Safety Standards (49 C.F.R. 571). This includes a further requirement that any transmission having more than one forward gear ratio must provide an additional gear position to enable engine braking. Most modern applications include at least five positions for the transmission gear selector lever, i.e. Park, Reverse, Neutral, Drive, and Low. Vehicle manufacturers typically provide additional ranges to allow the operator more control over vehicle performance. In general, there is a discrete number of valid gear selector lever positions of which a subset corresponds to Forward Drive ranges.
All automatic transmissions, including those that are electronically-controlled, must be able to determine transmission gear selector lever position selected by an operator to effectively control operation of the transmission to satisfy the operator's request. A typical transmission control system includes a sensor operable to monitor position of the transmission gear selector lever, and a control module operable to decode the sensor information and command actuation of specific devices in the transmission in response thereto. Determination of transmission gear selector lever position is deemed safety-critical, as a fault may cause vehicle movement to be commanded in an unintended direction. To avoid vehicle movement in an unintended direction, various approaches have been used, including a mechanical/hydraulic interlock system, wherein the transmission gear selector lever operates a hydraulic valve that prevents Reverse position from being obtained when a Forward Drive position is selected and vice versa.
A transmission system having a gear selector lever decoupled from the hydraulic system, such as in a shift-by-wire system used in an electric/fuel hybrid system, is precluded from employing such a mechanical/hydraulic interlock. Another mechanism is required to prevent a potentially incorrect reading of transmission gear selector lever position. One form of hybrid transmission system uses an architecture having identical clutching and power flow for both Forward and Reverse ranges, utilizing a shift-by-wire architecture that does not lend itself to traditional methods of differentiating between Forward and Reverse. Implementing a mechanical/hydraulic interlock system may be impractical due to an inability to provide the needed functionality and packaging constraints.
Prior art systems that have been offered to address the aforementioned concern include use of a manual valve with hydraulic redundancy to prevent inappropriate direction; use of dual analog sensors to determine gear selector lever position; use of fault detection using comparison and thresholding; use of digital encoding of gear selector lever position using multiple identical encoders and using a voting type architecture based on majority vote; and, use of a separate control module dedicated to reading and providing secure gear selector lever position.
Therefore, what is needed is a system that executes a command for vehicle propulsion and direction that is consistent with an operator's intent, and provides redundancy in the event that a controller becomes non-functional, especially in a distributed controller architecture.